#include <bits/stdc++.h>
using namespace std;

#define int int64_t
#define endl "\n"

using PII = pair<int, int>;
using TII = tuple<int, int, int>;
template<class T> using V = vector<T>;
using i64 = long long;
using i128 = __int128;
using u64 = unsigned long long;

#define FOR(i, a, b) for (int i = (int)(a); i <= (int)(b); i++)
#define FOR2(i, a, b, c) for (int i = (int)(a); i <= (int)(b); i += c)
#define FORD(i, b, a) for (int i = (int)(a); i >= (int)(b); i--)
#define FORD2(i, b, a, c) for (int i = (int)(a); i >= (int)(b); i -= c)
#define ALL(a) a.begin(), a.end()
#define RALL(a) a.rbegin(), a.rend()
#define lowbit(x) ((x) & (-x))
#define RE return;
#define Yes cout << "Yes" << endl;
#define YES cout << "YES" << endl;
#define No cout << "No" << endl;
#define NO cout << "NO" << endl;
#define pb push_back
#define eb emplace_back
#define fi first
#define se second
#define sz(x) (int)(x).size()

struct Pre {
    static constexpr int N = 1e5 + 10;
    V<int> factor[N];

    void init() {
        int mx = N - 10;
        FOR(i, 1, mx) {
            FOR2(j, i, mx, i) {
                factor[j].eb(i);
            }
        }
    }

} A;

void solve() {
    int n, m;
    cin >> n >> m;
    V<int> w(n + 1, 0);
    FOR(i, 1, n) cin >> w[i];

    sort(ALL(w));

    int ans = 1e18, r = 0, tot = m;
    map<int, int> mp;

    auto add = [&](int x) {
        for (auto t : A.factor[x]) {
            if (t > m) break;
            if (!mp[t]) tot--;
            mp[t]++;
        }
    };

    auto del = [&](int x) {
        for (auto t : A.factor[x]) {
            if (t > m) break;
            mp[t]--;
            if (!mp[t]) tot++;
        }
    };

    FOR(l, 1, n) {
        while (r + 1 <= n && tot != 0) add(w[++r]);
        if (tot > 0) break;
        ans = min(ans, w[r] - w[l]);
        del(w[l]);
    }

    if (ans == 1e18) ans = -1;

    cout << ans << endl;
}

signed main() {
    int Task = 1;

    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> Task;
    A.init();
    while (Task--) {
        solve();
    }

    return 0;
}
